This invention relates to apparatus for melting heat softenable materials and more particularly, to stream feeders or bushings made of platinum, rhodium or other precious metal alloys for producing continuous glass fibers.
Several methods have heretofore been employed in processing glass for forming continuous glass filaments for fibers. One method involves the steps of melting glass in a comparatively large furnace, refining the glass in a refining chamber and forming the glass into spherical bodies or marbles. The glass marbles are subsequently delivered into a stream feeder which is electrically heated to remelt the glass to a viscosity at which the streams of glass may be flowed through orifices in the heater and attenuated into fibers.
The more common method used today is the direct melt process wherein glass batch is reduced to a molten state and refined in a furnace. The molten glass flows from the furnace along a forehearth channel through stream feeders disposed along the forehearth. The feeders are heated by electrical resistance to control glass viscosity. Then, the streams of glass are delivered through orifices in the feeders or bushings for attenuation into fibers.
Both of the above fiber forming processes employ stream feeders or bushings made of high temperature resistant metal alloys such as platinum or rhodium. The stream feeder is a metal container having orifices in its bottom wall through which streams of glass flow for attenuation into fibers. Often, orificed tips, or projections, are attached to the bottom wall through which the glass streams flow. Feeders have typically been manufactured from precut parts which are welded together by conventional fusion welding techniques. The side walls and the bottom walls are positioned against the end wall and welded thereto to form sharp, non-filleted, two-dimensional corners. In this construction, a sharp, non-filleted, three dimensional corner is formed where the side wall, bottom wall and end wall join. Terminals to which electrical bus bars are attached for supplying current through the feeder are welded to both ends of the box shaped feeder. The feeder is then heated by its own electrical resistance.
There are a number of problems with this type of feeder construction. The welding together of many pieces is time consuming and the uniformity of welds can vary. Because of the nature of the fusion weld bead as compared to the sheet material, resistivity is changed through the weld zone and the same heat pattern may be difficult to stabilize or reproduce from feeder to feeder. This can be a particular problem at the sharp corners where one sheet of metal is laid upon another for welding. Also, the weld joining the terminal to the end wall is very important to the feeders performance. If this weld's electrical resistivity varies from feeder to feeder, the feeders will perform differently because of different heating characteristics. Also, if the weld joining the terminal to the end wall is a poor weld, the terminal can burn off from the feeder by electrical arcing causing premature failure of the feeder. The sharp non-filleted corners in this feeder construction can give rise to areas of high stress concentration. An area of high stress concentration at a poor weld joint can cause a leak in the feeder, and thus, premature failure of the feeder.